Baby is written in the key of B Major.
According to the Theorytab database, it is the 11st most popular key among Major keys and the 18th most popular among all keys. Major keys, along with minor keys, are a common choice for popular songs. The three most important chords, built off the 1st, 4th and 5th scale degrees are all major chords (B Major, E Major, and F♯ Major).
See the B Major Cheat Sheet for popular chords, chord progressions, downloadable midi files and more!